Method and device for sorting materials

ABSTRACT

To sort materials, in particular plastic parts, the items are carried at known conveying speed past a material recognition system which uses non-contact scanning, for example NIR spectroscopy, of each item to determine its material type and delivers a signal that identifies the type of material, which signal is used in sorting the items according to material type. To ensure that the determination of material type is done at a spot on the item that is not disturbed by a label, metal stamp or the like, the items are also conveyed past an imaging system which takes pictures of the items from which, using electronic image-processing techniques, features of colour and/or shape of the items are determined, from which in turn position data are derived about spots on the item at which an undisturbed determination of material type is possible. With the aid of these position data the determination of material type is then confined to such undisturbed spots.

The invention relates to a method for sorting materials, in particularplastic parts.

For the separation of plastics for recycling purposes, non-contact,optical methods based on NIR spectroscopy are known. The plastic partsare illuminated with a source of light in the near infra-red range, andthe reflectance spectrum is obtained. This shows distinct differences,on the basis of which separation according to material types can takeplace. Known apparatuses of this kind detect a passing plastic part,such as a bottle, a cup or similar with one or several measuring spotsand calculate the NIR spectrum from the light reflected by theilluminated measuring spot. However, a precondition is that themeasuring spot is not disturbed by a label, metal stamp or similarstick-on labels which will falsify the reflectance spectrum. Sharp edgescan also render the measuring result incorrect or uncertain. Similarnon-contact sensors based on microwaves or X-radiation are known, butare subject to the same limitations.

On the other hand, an optical imaging system for sorting plastic partsby colour, based on the use of colour video cameras, is known. In thiscase, pictures of the plastic parts conveyed past are taken, either inincident or transmitted light, using a colour video camera, and the formand/or colour is classified by an image computer. An image computersuitable for this is already known, for example, from the PCTpublication WO 90/10273, in which the classification of form and colouris described, using the automatic optical classification of plants as anexample. However, with this known system it is not possible todistinguish and separate plastic parts according to material type.

The object of the invention is to provide a method, which enablesmaterials to be sorted with reliable and undisturbed determination ofthe types of material and with a versatile selection of sortingcriteria, as well as a device for implementing the method.

To achieve this object, a method for sorting materials, in particularplastic parts, wherein the items are carried at known conveying speedpast a material recognition system, which uses non-contact scanning ofeach item within a measuring field to determine its material type anddelivers a signal that identifies the material type and is used insorting the items according to material type, is characterized accordingto the invention in that the items are conveyed past an imaging systemwhich takes pictures of the items from which, using electronicimage-processing techniques, features of colour and/or shape of theitems are detected, that from the features of colour and/or shapeposition data are derived regarding spots on the item at which anundisturbed determination of material type is possible, and that thedetermination of the material type by the material recognition system isconfined to such undisturbed spots as identified by the position data.

By linking the material type determination to the results of electronicimage processing the method according to the invention enables thematerial type of the items to be determined only at those spots whereundisturbed determination of the material type is possible. Moreover, itis also possible, in the case of items which contain components ofdifferent material types, to determine the material types of theindividual components separately. Finally, the features of colour and/orshape obtained by electronic image processing are available asadditional sorting criteria.

Advantageous configurations and further developments of the methodaccording to the invention and a device f or implementing the method arecharacterized in the sub-claims.

Further features and advantages of the invention ensue from thefollowing description of practical examples of embodiments shown in thedrawings. In the drawings

FIG. 1 shows a diagrammatic representation of a sorting system designedto implement the method according to the invention,

FIG. 2 shows a detailed view to explain the material type determinationin the sorting system of FIG. 1,

FIG. 3 shows a detailed view to explain a modified determination ofmaterial type,

FIG. 4 shows a modified embodiment of the sorting system of FIG. 1 and

FIG. 5 shows an extension of the sorting system of FIG. 1.

The sorting system 10 shown in FIG. 1 includes a conveyor belt 12, onwhich the items to be sorted 14 are placed, in such a way that there isno mutual contact. For the example it is assumed that the items 14 arehollow plastic bodies, such as bottles, cups or other containers, whichare either in their original state or can be deformed by compressing.The conveyor belt 12 is driven at a uniform speed in the direction ofthe arrow and is so designed that the items 14 are conveyed withoutslipping. For this purpose the conveyor belt 14 may have suitablestructures 16 which are only suggested schematically in the drawing.

The conveyor belt 12 passes through an imaging system 20, in whichcolour pictures of the items are taken and converted into video signals.The imaging system 20 includes a first colour video camera 22, to whichare allocated lamps 24, which illuminate the items 14 with diffuseincident light, so that the colour video camera 22 takes pictures of theitems 14 in incident light. The imaging system 20 also contains a secondcolour video camera 26, to which are allocated one or more lamps 28,which illuminate the items 14 from the opposite side of the conveyorbelt 12, so that the colour video camera 26 takes pictures of the items14 in transmitted light. To permit transmitted illumination, theconveyor belt 12 must be light-transmitting; for example, this can be anopen-pored fabric belt or it may have a latticed structure.

The colour video cameras 22 and 26 can be normal matrix or line cameras;they may also be special cameras with a spectral sensitivity adapted tothe items to be sorted. Each colour video camera emits electrical imagesignals at its outputs, which represent, for example, the colourseparations in the primary colours red, green and blue.

The image signals delivered by the two colour video cameras 22 and 26are delivered to an image computer 30, where features of colour andshape of the items photographed are obtained from the incident lightimage and the transmitted light image, using known methods of electronicimage processing. A suitable image computer, for example, is describedin detail in PCT publication WO 90/10273. This known image computerconverts the analog colour video signals delivered by a colour videocamera, image point by image point, into digital signals, thedigitalized colour video signals are classified, image point by imagepoint, according to specified colour classes and the digitalized andclassified colour video signals are finally stored, image point by imagepoint, in an image memory. The digitalized image stored in this way isfinally analyzed by the image computer on the basis of the allocation ofthe image points to various colour classes in order to obtain featuresof colour and geometric shape. A particular advantage of this knownimage computer consists in the fact that it can be trained by showingimage details to which is allocated a colour class in each case.

So that the features of colour and shape detected by the image computer30 can be allocated in the correct position to the items 14 and theircomponents, the image computer 30 receives data from a positiontransmitter 32 and a speed sensor 34. The position transmitter 32, forexample, always delivers a position signal to the image computer 30 whenan item 14 is located in a predetermined position and the speed sensor34 delivers to the image computer 30 a signal which indicates theconveying speed of the conveyor belt 12. By virtue of these signals andthe known distance between the position transmitter 32 and the colourvideo cameras 22 and 26, the image computer 30 can calculate theposition of each item 14 in the field of view of either colour videocamera 22 or 26 at the moment the picture is taken and can allocate aparticular position on the item to any colour or shape featuredetermined.

If necessary the image computer 30 can also trigger the taking of apicture when an item occupies a particular position in the field of viewof the colour video camera.

After passing through the imaging system 20 the items 14 are carried bythe conveyor belt 12 through a material recognition system 40. Thematerial recognition system 40 contains a contactless material sensor 42of known type, for example a microwave sensor, an X-ray sensor or aspectroscopic sensor operating in the near infrared range ("NIR"), and aradiation source 44 which irradiates the items 14 with radiation towhich the material sensor 42 used is sensitive. In the followingdescription it is assumed that the material sensor 42 is an NIRspectroscopic sensor; accordingly the source of radiation 44 illuminatesthe items 14 with light, the wavelength of which extends into the nearinfrared range. A normal halogen lamp can be used for this.

FIG. 2 shows how the NIR spectroscopic sensor 42 scans an item 14 thatis being carried by the conveyor belt 12 and illuminated by theradiation source 44, in order to determine the type of material of theitem 14. By way of a lens system 46, the NIR spectroscopic sensor 42picks up the light that is reflected by the surface of the item 14 in arelatively small measuring field 48. The measuring field 48, which isdetermined by the aperture of the lens system 46, typically has adiameter of approx. 2 cm. The light absorbed by the lens system 46 isdispersed into a spectrum by a grating, the wave length range beinglimited to between 1600 nm and 2000 nm. In the reflectance spectrum, theindividual plastics reveal clear differences, so that by makingcomparison with a library of previously established spectra, it ispossible to identify a specific plastic from them. The NIR spectroscopicsensor 42 emits a signal at the output which identifies the type ofmaterial detected or a signal which indicates that it was not possibleto determine the material.

In the example in FIG. 2 it is assumed that the item 14 is a deformedplastic bottle with a bottle body 50, a sealing cap 52, a label 54 andan opaque base cup 56. A base cup is a plastic casing to reinforce thebase of plastic bottles, said casing generally consisting of a plasticthat differs from the plastic of the bottle body. Similarly, the sealingcap generally consists of a material that differs from the bottle body.Correct determination of the type of material of a specific component ofthe item 14, for example the bottle body 50, assumes that the measuringfield 48 is located exclusively, at the moment of reading, on a surfaceregion of this component which must moreover not be disturbed by labels,metal stamps or other stick-on labels or print, which will falsify thereflectance spectrum. Sharp edges located in the measuring field 48 canfalsify the measurement result, too. If, for example, the measuringfield 48 is located on the label 54, then the measuring result may beuseless, if the label is made of paper and therefore does not supply anyrecognizable spectrum, or the measuring result may be wrong if the label54 consists of a plastic that is different from the bottle body 50. Ifthe measuring field at the moment of reading is partly located on thesealing cap 52 and partly on the bottle body 50, then it will not bepossible to determine positively the material type of either of thesetwo components. The same applies, if the measuring field 48 overlaps thebottle body 50 and the base cup 56. Even if the material sensor 42 hasperformed a perfect determination of the material type, it must beabsolutely clear whether the material type detected should be allocatedto the bottle body 50, the sealing cap 52, the label 54 or the base cup56.

These problems are solved in the case of the sorting system illustratedin FIG. 1 with the help of electronic image processing carried out bythe image computer 30. On the basis of the colour and shape featuresobtained, the image computer 30 can detect characteristic components ofthe items 14, such as the sealing cap, the bottle body, the label andthe base cup of the plastic bottle in FIG. 2 and supply position datawhich indicate the positions of the detected components in the storeddigitalized image. These position data are supplied to a materialrecognition computer 60 allocated to the material recognition system 40and is either converted by the latter or even by the image computer 30on the basis of the known conveying speed of the conveyor belt 12 andthe known distance between the imaging system 20 and the materialrecognition system 40, to positions which these components occupy whenpassing through the material recognition system 40. The materialrecognition computer 60 can then control the function of the materialsensor 42 in different ways, so that the material types of the items 14or the material types of their individual components are correctlyidentified.

A first possibility consists in that the material sensor 42 continuouslydetermines the material type whilst the items 14 are conveyed throughthe measurement window 18, but that the material recognition computer60, only selects those measuring results which originate from positionson the items, from which it is certain, based on the shape and colourfeatures obtained by the image computer 30, that they are appropriatefor undisturbed determination of material type. Based on the positiondata supplied by the image computer 30, the material recognitioncomputer 60 knows when the measuring field 48 of the material sensor 42is located at such an appropriate position on an item. In this way, itis also possible to determine separately the material types of variouscomponents of an item 14. A rapid-measuring NIR spectroscopic sensor,which is currently available, can deliver between 10 and 1000measurements per second. Based on the position data supplied by theimage computer 30, the material recognition computer 60 in the exampleshown in FIG. 2, can specifically select from the large number ofmeasurements those which originate only from the sealing cap 52, onlyfrom the bottle body 50, only from the label 54, or only from the basecup 56. This permits unmistakable allocation of the material typesdetermined by the material sensor 42 to the various components of theitem 14.

A modification of this first possibility consists in that thedetermination of the material type by the material recognition sensor 42is not continuous, but is always triggered by the material recognitioncomputer when the measuring field is located at the point required fordetermination of the material type.

A second possibility for undisturbed determination of the material typesusing the material recognition computer 60 , consists in that themeasuring field 48 of the material sensor 42 is directed towards a spotwhich permits undisturbed determination of the material type. This maybe done, for example, with the modified embodiment of the materialrecognition system 40 shown in FIG. 3. With this embodiment the lightbeam reflected by the surface of the item 14 in the measuring field 48is not received directly by the lens system 46 of the material sensor 42but following deflection by the mirror 62 of a mirror galvanometer 64.The material recognition computer 60 controls the mirror galvanometer 64in such a way that the measuring field 48 of the mirror 62 is directedtowards a specific spot on the item 14, of which it is established, byvirtue of the colour and shape features determined by the image computer30, that it is appropriate for undisturbed determination of the materialtype, and the position of which is known by virtue of the position datasupplied by the image computer 30. If required, the mirror galvanometer64 of the material recognition computer 60 can also be controlled sothat the measuring field 48 follows the chosen spot on the item 14 for aspecific period, whilst the latter is in motion.

Instead of deflecting the measuring field 48 with the help of a mirrorgalvanometer 64, it would, of course, also be possible to swing theentire material sensor 42 under control of the material recognitioncomputer 60 in such a way that the measuring field 48 is directedtowards the desired spot. Deflection with the help of a mirrorgalvanometer, however, produces the advantage that it can be done veryquickly and practically inertia-free with little power.

Higher spatial and temporal resolution material recognition can beachieved in that the normally circular aperture of the material sensor42 is deformed in such a way by anamorphotic lenses or optical fibresystems, that it takes on a linear shape directed transversely of thedirection of conveying. This means that, seen in the direction ofconveying, narrow segments can be measured. By means of a temporal orspatial control of this aperture based on the signals supplied by theimaging system 20, determination of the type of material, which isbetter limited in terms of location, is achieved than when a circularaperture is used. Thus, for example, the type of material of sealingcaps or other components of small dimensions can be specificallydetermined without overlapping the bottle body.

In particular when determining the types of material of items whichconsist of several different plastics, it may also be of advantage todivide the aperture of the material sensor in such a way, that severalseparate measuring fields are created which are then simultaneouslytaken in. Such a division is easy to perform with the help of anarrangement of optical fibres or by means of optical elements, such aslenses or masks. The division of the measuring field and the taking inof the desired spots by the part measuring fields obtained, can becontrolled by the material recognition computer 60, on the basis of datasupplied by the image computer 30. This means, for example, that in thecase of a beverage bottle, the body of which is made of a plastic andwhich is partially hidden by a label made from a different plastic, thebottle body can be detected simultaneously by two separate measuringfields on the left and right of the label, without the label also beingdetected.

In stead of illuminating a large area of the items 14 with the radiationsource 44, and determining the shape and extent of the measuring field48 through the aperture of the material sensor 42, as was previouslyassumed, it is also possible to design the material sensor 42 with alarge aperture and to determine the size, shape and position of themeasuring field by illumination which is limited in terms of locationand time, from the radiation source 44. Then, at the time ofmeasurement, only those points are actively illuminated wherematerial-type determination is to take place. This is achieved byillumination that is controlled by the material recognition computer 60,the light distribution of said illumination being determined on thebasis of the position data supplied by the image computer 30. Suitablecomputer-controlled light valves can, for example, be made up usingliquid crystal light valves, and these are known to the expert. Usingthis light-controlled development of the measuring fields, theadditional measures described above, for altering the shape of ordividing the measuring field, can be carried out in a way that isparticularly simple. For example, the division of the measuring fieldinto several partial measuring fields can be obtained by providing thatthe illumination takes place in a corresponding local pattern.

In the sorting system of FIG. 1, both the colour, shape and positiondata supplied by the image computer 30 and the material type datasupplied by the material recognition computer 60, are passed to asorting computer 70, which sorts the items 14 on the basis of thesedata. A series of sorting points 72 is arranged along the conveyor belt12, which, in the simplest case, could be made from pneumatic ejectorswhich blow the items 14 into separate containers down chutes 74. For thesake of simplification, only three such sorting points are shown in FIG.1, but their number can, of course, be as large as required. Eachsorting point 72 is used to separate out the items 14, which belong to aspecific sorting category. The user can set the sorting criteria foreach sorting category at the sorting computer 70 and can allocate eachsorting category to a sorting point 72. On account of the knownconveying speed and the known distances between the sorting points 72and the position transmitter 32, the imaging system 20 and the materialrecognition system 40, the sorting computer 70 can determine exactlywhen a specific item 14 can be found at the site of a specific sortingpoint and it can then trigger that sorting point 72 which is allocatedto the sorting category, to which this item belongs, according to thecolour and/or shape features supplied by the image computer 30, and inaccordance with the material type data supplied by the materialrecognition computer 60. Items which do not belong to any set sortingcategory, drop into a collecting bin at the end of the conveyor belt.

In this way, it is possible to achieve as fine a degree of sorting as isrequired in accordance with a large number of sorting criteria. Thus,not only can all items be sorted separately according to material types,but sorting can take place within each type of material or withinspecific material type groups according to transparent andnon-transparent materials, coloured and uncoloured materials, andfinally according to colours. Distinguishing between transparent andnon-transparent substances is made possible in particular by the factthat pictures are taken of the items both in incident and in transmittedlight.

FIG. 4 shows a modified embodiment of the sorting system of FIG. 1. Itcontains the same components as that of FIG. 1, which are alsodesignated by the same reference symbols as in FIG. 1. The sortingsystem of FIG. 4 differs from that of FIG. 1 in that the materialrecognition system 40, in the direction of travel of the conveyor belt12, is not arranged behind, but ahead of the imaging system 20. In thiscase, the material sensor 42 has to be operated in such a way that itcontinuously determines the types of material of all items which passthrough the measuring field 48, and the material recognition computer 60must have a memory in which all the measuring results supplied by thematerial sensor 42 are stored with allocation to the positions of thecomponents measured. Such a memory can, for example, be executed byelectronic time-delay elements. Subsequently, the image computer 30again determines colour and shape features and determines the positionof spots on the items, which are appropriate for undisturbeddetermination of the type of material. The position data supplied by theimage computer 30 is again conveyed to the material recognition computer60 and can be converted on the basis of the known conveying speed of theconveyor belt 12 and the known distance between the material recognitionsystem 40 and the imaging system 20 to the positions which the spotsconcerned had occupied previously whilst the type of material was beingdetermined in the material recognition system. Since the measuringresults are stored in the memory of the material recognition computer60, as allocated to these positions, this memory can be addressed by theconverted position data in such a way that the measuring results areread out which originate from the spots which were found to be suitableby the image computer 30 and identified by the position data. Thesemeasuring results are supplied to the sorting computer 70 in the sameway as for the sorting system of FIG. 1, and said sorting computer 70controls the sorting points 42 according to the set sorting criteria, inthe way described previously.

With the embodiment of FIG. 4, it is not possible to direct themeasuring field 48 of the material sensor 42 by deflecting it tospecific spots, on the basis of the position data supplied by the imagecomputer 30, as shown in FIG. 3, because this measure assumes that theimaging system is arranged ahead of the material recognition system.Apart from this, the embodiment in FIG. 4 offers the same facilities forsorting the items as that of FIG. 1.

FIG. 5 depicts a further embodiment of the sorting system, which showsin particular, how the data obtained by processing the images in theimage computer 30 can be beneficially used for further purposes. Theembodiment of FIG. 5 represents an extension of the embodiment of FIG.1; it contains all the components of the embodiment of FIG. 1, which aredesignated with the same reference symbols as in the latter. Inaddition, in the case of the embodiment of FIG. 5, two additionalstations, 80 and 90, through which the items 14 are conveyed, areinserted between the material recognition system 40 and the sortingpoints 72 along the conveyor belt 12.

Station 80 is used to separate out items made from glass, i.e. in thecase of the example chosen of sorting hollow bodies, in particular glassbottles and other glass containers. The weight of each hollow body 14 isdetermined by a rapid belt scale 82 and entered in a computer 84. Fromthe weight alone, it is not possible to distinguish between glass andplastic containers, if the dimensions of the hollow body are not known.Therefore, data regarding features of shape are transmitted by the imagecomputer 30 to the computer 84 and from these data, the computer 84 canestimate the size of the hollow body. By linking the weight andgeometrical size, the computer 84 decides whether the item in questionis a glass body or a plastic body. The computer 84 then controls asorting point 86 in such a way, that the glass bodies are separated byan eject device 88. The sorting point 86, as shown in FIG. 5, may belocated immediately next to station 80. However, it may also be added tothe sorting points 72 at the end of the conveyor belt 12; in this case,the computer 84 delivers to the sorting computer 70 data, which identifythe positions of recognized glass bodies, so that the sorting computer70 can separate out the glass bodies by operating the appropriatesorting point. As a rule, it is advantageous to combine all the ejectstations at the end of the conveyor belt and to set them up in closeproximity to one another. Synchronization is easily achieved by thesorting computer 70, since from the position data supplied by thevarious stations, the known belt speed and the known distances betweenthe stations and the sorting points, the moment when the sorting pointsare to be operated can be precisely determined.

The station 90 is a separating station, where the sealing caps 52 andthe base cups 56 are separated from the bottle bodies 50 and ejected. Acomputer 92 receives position data from the image computer 30 regardingthe positions of sealing caps 52 and base cups 56 on the hollow bodies50, and from the material recognition computer 60 data about the typesof material of these components. On the basis of these data, thecomputer 92 decides whether a sealing cap and/or a base cup is to beseparated, and at the right moment when the hollow body 50 is passing,it triggers a separation device 94, which separates the part in questionand ejects it via an eject station 96.

It can therefore be seen that in the additional stations 80 and 90, asin the material recognition system 40, the features of shape and colourand position data obtained by image processing in the image computer 20are also used for concerted sorting.

Naturally, additional stations can be added to the sorting system ofFIG. 5, if required. A particular advantage of the invention consists inthe fact that the sequence of stations is arbitrary, since the functionsof the stations are controlled on the basis of the position data whichcan be calculated for each station independently of the others.

Further modifications, with which the expert is familiar, can of coursebe made to the sorting systems described. For example, instead of aconveyor belt, any other known means of conveying may be used which iscapable of conveying the items through the various stations withoutslipping, for example, a rotating table. Furthermore, the functions ofvarious computers which are shown separately in the drawings for thesake of clarity, may be carried out by one common computer.

I claim:
 1. A method for sorting materials, in particular plastic parts,comprising:conveying items at known conveying speed past a materialrecognition system which uses non-contact scanning of each item todetermine its material type in a measuring field and delivers a signalthat identifies the material type and is used in sorting the itemsaccording to material type; conveying the items past an imaging systemwhich takes pictures of the items; detecting features of colour and/orshape of the items from the pictures using electronic image processingtechniques; deriving position data from the features of colour and/orshape regarding spots on the item at which an undisturbed determinationof the material type is possible without disturbance by other materialtypes; and determining the material type using the material recognitionsystem, wherein determination of the material type is confined to suchundisturbed spots as identified by the position data.
 2. The methodaccording to claim 1, wherein the imaging system in the direction ofconveyance of the items is located ahead of the material recognitionsystem.
 3. The method according to claim 2, wherein determination of thematerial type is triggered by the material recognition system when anundisturbed spot of an item, identified by the position data, is locatedin the measuring field of the material recognition system.
 4. The methodaccording to claim 2, wherein the position data are used for controllinga deflection system, so that the measuring field of the materialrecognition system is directed towards an undisturbed spot on the item.5. The method according to claim 2, wherein the illumination of theitems by radiation necessary to determine the material type, is limitedto the undisturbed spots identified by the position data while themeasuring field of the material recognition system covers a larger areaof the items.
 6. The method according to claim 2, wherein of all thesignals supplied by the material recognition system, only those areutilized as being valid which originate from undisturbed spotsidentified by the position data.
 7. The method according to claim 1,wherein the material recognition system, in the direction of conveyanceof the items, is located ahead of the imaging system, the signalssupplied by the material recognition system are stored and of the storedsignals only those are utilized as being valid, which are allocated tothe undisturbed spots as identified by the position data.
 8. The methodaccording to claim 1, wherein the measuring field of the materialrecognition system is deformed by optical systems in such a way that, inthe direction of conveyance, it has a high local resolution comparedwith the transverse direction.
 9. The method according to claim 1,wherein the measuring field of the material recognition system isdivided by optical elements and/or fibre optics in such a way that itcovers the items at various spots according to a spatial pattern. 10.The method according to claim 9, wherein the spatial pattern isdetermined by the distribution of the illumination of the items usingradiation necessary to determine the material type within the measuringfield.
 11. The method according to claim 9, wherein the spatial patternis selected in such manner that it simultaneously detects several spotson the items which are significant for determining the material type.12. A device for sorting materials, in particular plastic parts,comprising a conveying device for the slip-free conveyance of the itemsat a known speed, an imaging system past which the items are conveyedand which takes pictures of the items and converts these into electricalimage signals, an image computer which determines features of colourand/or shape of the items from the electrical image signals, a materialrecognition system past which the items are conveyed and which containsa material sensor for non-contact scanning of each item in a measuringfield and a material recognition computer which by processing the outputsignals of the material sensor determines the material type in themeasuring field and delivers a signal that identifies the material type,and a sorting device which sorts the items on the basis of the signalssupplied by the imaging system and by the material recognition systemaccording to fixed sorting categories, wherein the image computerderives, from the features of colour and/or shape obtained, positiondata about spots on the items at which an unequivocal determination ofmaterial type is possible without disturbance by other material types,and wherein the material recognition computer receives the position datafrom the image computer and, based on the position data received,confines the determination of the material type to the undisturbed spotsidentified by the position data.
 13. The device according to claim 12,wherein the imaging system contains a colour video camera which takespictures of the items in incident light.
 14. The device according toclaim 12, wherein the imaging system contains a colour video camerawhich takes pictures of the items in transmitted light.
 15. The deviceaccording to claim 14, wherein each colour video camera is a line ormatrix camera.
 16. The device according to claim 12, wherein thematerial sensor is an NIR spectroscopic sensor.
 17. The device accordingto claim 12, wherein the imaging system, in the direction of conveyanceof the items, is located ahead of the material recognition system. 18.The device according to claim 17, wherein an optical deflection deviceis located in the path of the light beam that determines the measuringfield, said deflection device being controlled by the materialrecognition computer on the basis of the position data supplied by theimaging system in such a way that the measuring field is directed to anundisturbed spot on the item, as identified by the position data. 19.The device according to claim 18, wherein the optical deflection deviceis a mirror galvanometer.
 20. The device according to claim 12, whereinthe imaging system, in the direction of conveyance of the items, islocated behind the material recognition system and the materialrecognition computer contains a memory in which all the signals suppliedby the material sensor are stored and which is read out by addressing itby the position data supplied by the image computer.
 21. The deviceaccording to claim 12, wherein a sorting computer is assigned to thesorting device and this computer receives data from the image computerand from the material recognition computer and controls the sorting ofthe items, on the basis of these data, according to sorting categoriesdetermined by set sorting criteria.
 22. The device according to claim12, comprising a station located on the conveying device for separatingout glass bodies using a belt scale measuring the weight of the items,and an associated computer which receives data from the image computerabout shape features, which enable the size of the items to bedetermined, and which detects present glass bodies on the basis of thesize and weight measured.
 23. The device according to claim 12,comprising a station located along the conveying device for separatingcomponents of the items using a separating device, and a computercontrolling the separating device, which receives position data from theimage computer about the components to be separated.